Lithium ion batteries have a high capacity and a high energy density, and can be easily made compact and lightweight. Accordingly, lithium ion batteries are widely used as a power source for compact portable electronic devices such as cellular phones, personal digital assistants (PDAs), notebook personal computers, digital cameras and portable game consoles. In addition, application development as a power source for vehicles, such as hybrid vehicles, and as an uninterruptible power supply is also in progress for lithium ion batteries.
Currently typical lithium ion batteries employ graphite as a negative electrode active material. On the other hand, attempts have been made in recent years to use active materials that are capable of absorbing and releasing lithium by alloying and dealloying (alloy-formable active materials) as a new negative electrode active material for achieving an even higher capacity. As preferred examples of such alloy-formable active materials, a substance simply composed of silicon or tin, and oxides and alloys containing silicon or tin are known.
However, alloy-formable active materials expand and contract significantly during absorption and release of lithium, and causes a relatively large amount of stress when expanded. Accordingly, the repetition of charge and discharge can cause cracking in the negative electrode active material layer containing an alloy-formable active material, and a new surface coming into contact with the electrolyte solution (hereinafter referred to as a “fresh surface”) can easily appear inside the negative electrode active material layer. Such fresh surfaces can cause side reactions other than charge and discharge reactions due to contact with the electrolyte solution, producing by-products such as gas and a coating that covers the negative electrode active material layer. The gas and coating produced by such side reactions can cause deformation of the electrode and a reduction in the service or cycle life. Moreover, the electrolyte solution contained in the battery is consumed by the side reactions, causing a reduction in charge/discharge cycle characteristics.
Patent Document 1 discloses a negative electrode made of an alloy-formable active material in which the surface is covered with a polymer film. The polymer film is formed from a mixed solution of a crosslinking monomer, a polymer material and an organic solvent. The pores within the negative electrode active material layer are filled with a cross-linked form of the monomer that is capable of conducting ions but has a low ion conductivity level. The crosslinking monomer can be polyethylene glycol dimethacrylate or the like, and the polymer material can be polymethyl methacrylate or the like.
Patent Document 2 discloses a lithium battery including an alloy-formable negative electrode active material, a first binder coating layer that is formed on the surface of the negative electrode active material, a second binder coating layer that is formed on the surface of the first binder coating layer, and a current collector. The first binder coating layer is made of an elastic polymer binder such as styrene-butadiene rubber. The second binder coating layer is a layer for binding the negative electrode active material layer and the current collector, and is made of a cellulose (e.g., carboxymethyl cellulose), a polystyrene glycol, a poly(N-vinyl amide), a polyacrylamide, or the like.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-197258
Patent Document 2: Japanese Laid-Open Patent Publication No. 2007-80827